1. Field of the Invention
This invention relates to systems which utilize received signals from satellite positioning systems (SPS) to determine navigational information (e.g., position, time, distance, velocity, etc.).
2. Background of the Invention
SPS receivers such as GPS (Global Positioning Satellite) system receivers normally determine their position by computing relative times of arrival of signals transmitted simultaneously from a multiplicity of satellites such as GPS (or NAVSTAR) satellites. These satellites transmit, as part of their satellite data message, both satellite positioning data as well as data on clock timing, so-called xe2x80x9cephemerisxe2x80x9d data. In addition they transmit time-of-week (TOW) information that allows the receiver to determine unambiguously local time. Each received GPS signal (in C/A mode) is constructed from a high rate (1.023 MHz) repetitive pseudorandom (PN) pattern of 1023 symbols, commonly called xe2x80x9cchips.xe2x80x9d Further imposed on this xe2x80x9cspreading sequencexe2x80x9d pattern is low rate data at a 50 Hz (or baud) rate. This data is the source of the above-mentioned TOW information.
Typically, an SPS receiver computes one or more xe2x80x9cpseudorangexe2x80x9d measurements, each of which represents the range between the receiver and a satellite vehicle (SV). The term xe2x80x9cpseudorangexe2x80x9d is generally used to point out that the range measurement may include error due to one or more factors, including, for example, the error between time as indicated by the clock of the SPS receiver and a reference time, such as the reference time associated with the more accurate atomic clock of the satellites. Thus, the SPS receiver typically uses the pseudoranges, along with timing and ephemeris data provided in the satellite signal to determine a more accurate set of navigational data, such as position, time, and/or range.
To complete computation of the navigational information, such as the position of the SPS receiver, the SPS receivers should generally process the 50 baud data. To process the 50 baud data, the satellite signal level received by the SPS receiver should be sufficiently strong. Thus, conventional SPS receivers may not be able to completely determine navigational information, such as their position and/or time, if the received signal level and/or signal-to-noise ratio (SNR) is too low. Unfortunately, in some situations, such as where there is blockage of the satellite signals, the received signal level from the GPS satellites may be too low to demodulate and read the satellite data signals without error. Such situations may arise in personal tracking and/or other mobile applications.
The present invention provides a signal processing method and apparatus for improving the sensitivity of an entity, such as a mobile satellite positioning system (SPS) receiver, to better enable processing of the satellite message and determination of navigational information, such as time, position, etc., in mobile and/or other environments wherein noise and/or other factors may diminish signal strength, SNR, etc.
A method and apparatus for processing satellite positioning system (SPS) signals is provided. In one embodiment, a SPS receiver receives at least two signal samples representing, at least in part, common information, wherein the two signal samples are associated with one or more satellite messages. By combining the two signal samples, navigational information contained in the samples (e.g., time, position, velocity, etc.) may be determined based on the combination of the two signal samples. According to another embodiment, the two signal samples are differentially demodulated and summed together to form the combination.